ATX PSU as bench power supply. Voltage dropping issue

Question

I am trying to use a PC ATX PSU as a bench power supply in order to achieve a high current 5V and 12V supply. The PSU is 550W and I am using a single SATA power connector (to connect to the device to be powered). I also have the PSU on signal tied to ground permanently so the switch on the back of the PSU is the main method of turning it on and off.

When connecting a load to either the 12V rail or the 5V rail, the voltage drops to outside the tolerance of the PSU. The load does not exceed the current rating of the PSU. For example, when I connect a 1 ohm load across 5V and 0V, the supply voltage drops to 3.7V. The wiring used to power the device from the SATA connector is suitable enough to handle the required current.

Is it possible that the wiring from the PSU PCB to the SATA connector is causing the voltage drop (too small to handle the current I require)? If this is not likely, can anyone offer some insight into what I am seeing? Also, different load values have been put across both the 12V and 5V rails (10 ohms to 1.8k ohms) to try and force the PSU into thinking a motherboard is attached (this has not changed anything).

Answer 1

Unfortunately, there is not a solid standard for the ATX supplies as they evolved over time.

You may need to load all three buses. Try 1A on the 12 volt, 0.5A on the 3.3V, and 0.3A on the 5 volt bus. Older supplies even required that the loads on the 5 and 3 volt buses total to be approximately equal to the 12 volt bus to work properly.

You may have a supply which requires a 3.3V sense line to be connected to the 3.3V output to function. This is usually identified with a 20 pin connector. Connect pin 11 to the 3.3 volt lines if this is the case.